The present invention relates to a process for passivating pyrophorous solids, especially catalysts, and the thus prepared solids themselves.
The passivation of supported nickel catalysts is known. Existing processes rely in many cases on a partial oxidation of the metal surface. DE-B-1 299 286, DE-A-2 209 000 and DE-A-2 530 818 describe passivation processes in aqueous solution. The oxidizing agents used are hydrogen peroxide, hypochlorites or oxygen. The processes described, however, are not universally useful, since there are many applications where the water has to be removed before use of the catalyst.
U.S. Pat. No. 2,565,347, U.S. Pat. No. 3,868,332, DD 156 169, DD 156 345, DD 156 347, DD 157 161 and RO 68 600 disclose the passivation of pyrophorous nickel-containing catalysts with oxygen-containing nitrogen. In the processes described, the catalyst is purged with nitrogen prior to passivation to desorb hydrogen from the catalyst surface. The disclosed processes differ from each other with regard to the temperature settings for the desorption and the passivation. One of the disadvantages of the processes described is that they provide, when practiced on an industrial scale, non-uniform products and require long passivation times.
EP-A 89 761 describes the passivation of a nickel-Al2O3 catalyst by treating the catalyst with CO2 at temperatures of 175 to 200xc2x0 C. for a period of at least 30 minutes and subsequent cooling in CO2 to ambient temperature.
U.S. Pat. No. 4,090,980 discloses the passivation of a catalyst with an oxygen-nitrogen mixture after prior treatment with CO2, which comprises treating the reduced catalyst with an inert gas by circulating the inert gas through it at a catalyst temperature of 149xc2x0 C., then cooling the catalyst to a temperature of 10 to 38xc2x0 C., then progressively increasing the CO2 content in the circulating gas to 80% by volume and subsequently adding oxygen to obtain a concentration of 0.05% by volume. The treatment with this mixture is carried on until 25% of the monolayer of the catalyst surface is coated with oxygen. The oxygen concentration is then raised to 1% by volume and, after the monolayer on the catalyst surface has been completely formed by oxygen species, the oxygen content is slowly raised and the CO2 content of the gas mixture reduced.
A combined CO2 and O2 treatment is also disclosed in SU 1 344 404. In this passivation process, the catalyst is treated with CO2 at temperatures of 250 to 300xc2x0 C., subsequently in the CO2xe2x80x94O2 mixture with oxygen concentrations of 0.4 to 2% by volume at temperatures of 100 to 250xc2x0 C. for a period of 20 minutes, the catalyst is cooled down to ambient temperature under CO2 and subsequently air is flowed through the catalyst bed.
DE 3 629 631 describes the passivation of nickel-containing catalysts with CO2, steam or oxygen. When the passivation has been carried out with CO2 in the temperature range from 25 to 250xc2x0 C., this disclosure requires that the CO2 treatment be followed by a further passivation with oxygen-nitrogen mixtures.
In summary it can be stated that existing passivation processes require very long passivation times, especially to passivate with oxygen-nitrogen mixtures, and so are very costly and, what is more, provide nonuniformly passivated catalysts.
It is an object of the present invention to provide a process for passivating pyrophorous solids, especially pyrophorous catalysts, that leads to more uniformly passivated solids, especially catalysts, at low cost.
This object is achieved by a process for passivating a preferably reduced and/or preferably inertized pyrophorous solid, especially a catalyst, particularly preferably a supported metal catalyst, wherein the catalyst a) is treated in a CO2xe2x80x94N2 gas mixture having a CO2 content of 0.5 to 10% by volume at temperatures of 91xc2x0 C. to 350xc2x0 C. for at least 30 minutes, b) then cooled down to a temperature of not more than 90xc2x0 C. in the CO2xe2x80x94N2 gas mixture of step a), c) after reaching the temperature mentioned in step b) oxygen, preferably air, is added to the CO2xe2x80x94N2 gas mixture in a first passivation phase up to a content of 0.2 to 1.5% by volume and the catalyst is treated in the CO2xe2x80x94N2xe2x80x94O2 gas mixture for at least 30 minutes by shaking, d) and then in a second passivation phase, with shaking, the CO2 content in the CO2xe2x80x94N2xe2x80x94O2 gas mixture is lowered to  less than 0.1% by volume and the O2 content is raised to 1.5xe2x80x9421% by volume.
The process of the invention has the advantage of short stabilization times while at the same time providing readily reactivable catalysts possessing very good thermal stability. Advantageously, the catalysts are passivated particularly uniformly. It is in fact surprising that the treatment with low-CO2 inert gases under the stated conditions provides very uniformly and easily reactivable catalysts.
For the purposes of the present invention, a pyrophorous solid is a solid body that ignites, or tends to ignite, spontaneously, especially a body that will spontaneously combust in air when exposed to it in a state of very fine subdivision.
In a particularly preferred embodiment, the invention provides an aforementioned process wherein the pyrophorous solid is a pyrophorous supported metal catalyst. The invention accordingly provides for example supported metal catalysts wherein the metal component is nickel, cobalt, copper, iron, aluminum, zinc or mixtures or alloys of two or more thereof. In a preferred embodiment, the support component of the supported metal catalyst used according to the invention consists of or contains for example Al2O3, SiO2, SiO2 xc2x7Al2O3, TiO2, ZrO2, mixtures of these oxides, activated carbon, zeolites, clays, natural silicates or mixtures of two or more thereof. The invention also provides that the supports may have been chemically modified, for example by treatment with phosphate, sulfate, fluoride or chloride compounds. It will be appreciated that, according to the invention, it is also possible for the pyrophorous supported metal catalyst used to be doped, for example by additions of elements of the sixth to eight transition group of the Periodic Table of the Elements, such as platinum, palladium, rhodium, chromium, tantalum, titanium, iron or their mixtures etc.
It will be appreciated that the supported metal catalyst used according to the invention can additionally a contain additive materials such as molding assistants, lubricants, plasticizers, pore-formers, moisteners, etc.
The herein described process for passivating a solid starts with a reduced and inertized solid. The solid, especially a catalyst, can be reduced by treating the catalyst in a hydrogen stream at elevated temperatures. After reduction, the catalyst is, if appropriate, inertized in a nitrogen stream. The invention thus provides in a preferred embodiment that the solid, especially a catalyst, to be passivated is reduced and/or inertized before passivation. The reducing can be effected by treating the solid with a hydrogen stream at temperatures of 250 to 500xc2x0 C., a volume hourly space velocity of 250 to 300 v/v h and a heating rate of 50xc2x0 C./h to 200xc2x0 C./h, preferably at periods of 2 hours to 16 hours at reduction temperature. This can be followed by an inertization under the following conditions: at reduction temperature the hydrogen stream is switched over to a nitrogen stream, inertization is carried out at this temperature for about 30 minutes and this is followed by cooling down in the nitrogen stream to the temperature of the treatment with the N2xe2x80x94CO2 mixture, the N2 volume hourly space velocity being 250 to 3 000 v/v h.
The oxygen to be added during the first passivation phase of step c) can preferably be added by adding air up to the stated oxygen concentration. It will be appreciated, however, that the oxygen can also be added in pure form for example.
In a preferred embodiment of an aforementioned process provided according to the invention, the process according to the invention is continuously or batch operated in a catalyst bed, especially in a catalyst bed whose height to diameter ratio is in the range from 0.05 to 1.
In a further preferred embodiment, the concentration of the CO2 during the treatment with the CO2xe2x80x94N2 mixture as per the first step a) is 1 to 2.5% by volume.
In a further preferred embodiment, the volume hourly space velocity during the treatment with the CO2xe2x80x94N2 mixture as per the first step a) is 500 to 10 000 v/v h. In a further preferred embodiment, the volume hourly space velocity during the treatment with the CO2xe2x80x94N2 mixture as per the first step a) and/or during the treatment with the CO2xe2x80x94N2xe2x80x94O2 gas mixture as per the third and fourth steps c) and d) is 100 to 3 000 v/v h.
In a further preferred embodiment, the treatment in the CO2xe2x80x94N2xe2x80x94O2 gas mixture as per the third and fourth steps c) and d) is carried out for a period of more than 30 minutes, for example 33 minutes to 8 hours. Step d) can be carried out for a period of not less than 3 minutes, in a preferred embodiment.
In a further development of the aforementioned process, the ratio of the duration of the treatment as per the third step c), i.e., the first passivation phase, to the duration of the fourth step d), i.e., the second passivation phase, is 9:1.
In a further preferred embodiment, the temperature of the treatment of the catalyst with the CO2xe2x80x94N2xe2x80x94O2 gas mixture of step c) and/or d) is 50 to 70xc2x0 C.
In a further preferred embodiment, the CO2 concentration in the CO2xe2x80x94N2xe2x80x94O2 gas mixture during the treatment of the third step c) is 0.5 to 1.5% by volume. In a preferred embodiment, the CO2 content of the mixture from step a) can be lowered, for example to the aforementioned range, for the duration of step c).
In a further preferred embodiment of the present invention, there is provided an aforementioned process wherein the O2 concentration in the CO2xe2x80x94N2xe2x80x94O2 gas mixture during the treatment of the third step c) is 0.25 to 0.8% by volume.
In a further preferred embodiment of the invention, the O2 concentration during the treatment of the fourth step d) is 5 to 10% by volume.
In a further embodiment, the invention provides an aforementioned process wherein the shaking of the catalyst bed during steps c) and/or d) is effected at intervals of 10 to 20 minutes for a period of 0.5 to 2 minutes in each case. It is advantageous to employ shaking frequencies of 10 to 50 Hz.
It will be appreciated that it is also possible, especially in the case of pulverulent catalysts and catalysts possessing very high strengths, to agitate the catalyst bed by fluidization or disposition in a rotary tube oven. At any rate, it is an essential aspect of the present invention to agitate the catalyst in the oxygen-carbon dioxide-nitrogen mixture at least temporarily during the passivation phases of steps c) and d), for example in a moving bed.
The invention also provides a passivated solid, especially a passivated supported metal catalyst, prepared according to one of the subject processes. Such catalysts are notable for good reactivability, excellent thermal air stability and uniform passivation.